Hi hopeless ro.
I also wondered what UE stood for. From a search I came across a webpage in the 'Spaceline - Covering the Past, Present and Future of Cape Canaveral' website that had a JUNO 1 Fact Sheet. In this it stated:-
"Following scrubs on January 29 and January 30, a Juno I bearing the letters "UE" (Redstone #29; see Jupiter C) successfully launched Explorer I, the first U.S. satellite. The Juno I was launched from Cape Canaveral Launch Pad 26A at 10:48 p.m. EST on January 31, 1958.
This was followed by five more Juno I satellite launch attempts. A Juno I bearing the letters "UV" (Redstone #26) was launched from Cape Canaveral Launch Pad 26A on March 5, 1958. The fourth stage failed to fire in this unsuccessful attempt to launch Explorer II.
A Juno I bearing the letters "UT" (Redstone #24) was launched from Cape Canaveral Launch Pad 5 on March 26, 1958. This Juno I successfully carried Explorer III into orbit. A Juno I bearing the letters "TT" (Redstone #44) launched from Cape Canaveral Launch Pad 5 on July 26, 1958 successfully carried Explorer IV into orbit.
The last two Juno I launches were failures. A Juno I bearing the letters "TI" (Redstone #47) launched from Cape Canaveral Launch Pad 5 on August 24, 1958 failed to carry Explorer V into orbit when the Redstone booster bumped the upper stages, sending them out of control.
Finally, a Juno I bearing the letters "HE" (Redstone #49 should have been lettered "TE" according to code, but "HE" was used instead) was launched from Cape Canaveral Launch Pad 5 on October 22, 1958. The upper stage clusters broke off in an unsuccessful attempt to deploy the Beacon I balloon-type satellite.".
That does not tell us what for example UE stood for (and I have still to find out) but it seems that the letters (usually) were a code for the order that the modified Redstone ballistic missile were used for launches.
I hope that helps.